Downhole Formation Testing and Sampling Apparatus

ABSTRACT

Systems and methods for downhole formation testing based on the use of one or more elongated sealing pads disposed in various orientations capable of sealing off and collecting or injecting fluids from elongated portions along the surface of a borehole. The various orientations and amount of extension of each sealing pad can increase the flow area by collecting fluids from an extended portion along the surface of a wellbore, which is likely to straddle one or more layers in laminated or fractured formations. Various designs and arrangements for use with a fluid tester, which may be part of a modular fluid tool, are disclosed in accordance with different embodiments.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. patentapplication Ser. No. 13/842,507, filed Mar. 15, 2013, which is acontinuation-in-part of U.S. patent application Ser. No. 13/562,870,filed Jul. 31, 2012, now U.S. Pat. No. 8,522,870 issued Sep. 3, 2013,which is a continuation of U.S. patent application Ser. No. 12/688,991,filed Jan. 18, 2010, now U.S. Pat. No. 8,235,106 issued Aug. 7, 2012,which is a continuation of U.S. patent application Ser. No. 11/590,027,filed Oct. 30, 2006, now U.S. Pat. No. 7,650,937 issued Jan. 26, 2010,which is a continuation of U.S. patent application Ser. No. 10/384,470,filed Mar. 7, 2003, now U.S. Pat. No. 7,128,144 issued Oct. 31, 2006.The entire disclosure of these prior applications is incorporated hereinby this reference.

FIELD OF THE INVENTION

The present invention pertains generally to investigations ofunderground formations and more particularly to systems and methods forformation testing and fluid sampling within a borehole.

BACKGROUND OF THE INVENTION

The oil and gas industry typically conducts comprehensive evaluation ofunderground hydrocarbon reservoirs prior to their development. Formationevaluation procedures generally involve collection of formation fluidsamples for analysis of their hydrocarbon content, estimation of theformation permeability and directional uniformity, determination of theformation fluid pressure, and many others. Measurements of suchparameters of the geological formation are typically performed usingmany devices including downhole formation testing tools.

Recent formation testing tools generally comprise an elongated tubularbody divided into several modules serving predetermined functions. Atypical tool may have a hydraulic power module that converts electricalinto hydraulic power; a telemetry module that provides electrical anddata communication between the modules and an uphole control unit; oneor more probe modules collecting samples of the formation fluids; a flowcontrol module regulating the flow of formation and other fluids in andout of the tool; and a sample collection module that may contain varioussize chambers for storage of the collected fluid samples. The variousmodules of a tool can be arranged differently depending on the specifictesting application, and may further include special testing modules,such as NMR measurement equipment. In certain applications the tool maybe attached to a drill bit for logging-while-drilling (LWD) ormeasurement-while drilling (MWD) purposes. Examples of suchmultifunctional modular formation testing tools are described in U.S.Pat. Nos. 5,934,374; 5,826,662; 5,741,962; 4,936,139, and 4,860,581, thecontents of which are hereby incorporated by reference for all purposes.

In a typical operation, formation-testing tools operate as follows.Initially, the tool is lowered on a wireline into the borehole to adesired depth and the probes for taking samples of the formation fluidsare extended into a sealing contact with the borehole wall. Formationfluid is then drawn into the tool through inlets, and the tool canperform various tests of the formation properties, as known in the art.

Prior art wireline formation testers typically rely on probe-typedevices to create a hydraulic seal with the formation in order tomeasure pressure and take formation samples. Typically, these devicesuse a toroidal rubber cup-seal, which is pressed against the side of thewellbore while a probe is extended from the tester in order to extractwellbore fluid and affect a drawdown. This is illustrated schematicallyin FIG. 1, which shows typical components of an underground formationtester device, such as a probe with an inlet providing fluidcommunication to the interior of the device, fluid lines, various valvesand a pump for regulating the fluid flow rates. In particular, FIG. 1shows that the rubber seal of the probe is typically about 3-5″ indiameter, while the probe itself is only about 0.5″ to 1″ in diameter.In various testing applications prior art tools may use more than oneprobe, but the contact with the formation remains at a small point area.

The reliability and accuracy of measurements, made using the toolillustrated in FIG. 1, depends on a number of factors. In particular,the producibility of a hydrocarbon reservoir is known to be controlledby variations in reservoir rock permeability due to matrixheterogeneities. It is also well known that underground formations areoften characterized by different types of porosity and pore sizedistribution, which may result in wide permeability variations over arelatively small cross-sectional area of the formation. For example,laminated or turbidite formations, which are common in sedimentaryenvironments and deep offshore reservoirs, are characterized by multiplelayers of different formations (e.g., sand, shale, hydrocarbon). Theselayers may or may not be aligned diagonally to the longitudinal axis ofa vertical borehole and exhibit differing permeabilities and porositydistributions. Similarly, as shown in FIG. 2, in naturally fracturedformations whose physical properties have been deformed or alteredduring their deposition and in vugular formations 53 having erratic poresize and distribution, permeabilities to oil and gas may vary greatlydue to the matrix 55 heterogeneities.

For example, in laminated or turbidite reservoirs, a significant volumeof oil in a highly permeable stratum, which may be as thin as a fewcentimeters, can be trapped between two adjacent formation layers, whichmay have very low permeabilities. Thus, a formation testing tool, whichhas two probes located several inches apart along the longitudinal axisof the tool with fluid inlets being only a couple of centimeters indiameter, may easily miss such a rich hydrocarbon deposit. For the samereasons, in a naturally fractured formation, in which oil or gas istrapped in the fracture, the fracture, such as fracture 57 shown in FIG.2, acts as a conduit allowing formation fluids to flow more freely tothe borehole and causing the volume of hydrocarbon to be underestimated.On the other hand, in a vugular formation a probe may encounter an oilvug and predict high volume of hydrocarbon, but due to the lack ofconnectivity between vugs such high estimate of the reservoir'sproducibility will be erroneous.

One solution to the above limitations widely used in prior art wirelineformation testers is to deploy straddle packers. Straddle packers areinflatable devices typically mounted on the outer periphery of the tooland can be placed as far as several meters apart from each other. FIG. 3illustrates a prior art device using straddle packers (cross-hatchedareas) in a typical configuration. The packers can be expanded inposition by inflating them with fluid through controlled valves. Whenexpanded, the packers isolate a section of the borehole and samples ofthe formation fluid from the isolated area can be drawn through one ormore inlets located between the packers. These inflatable packers areused for open hole testing and have historically been deployed on drillpipe. Once the sample is taken, the straddle packers are deflated andthe device can be moved to a new testing position. A number of formationtester tools, including the Modular Formation Dynamics Tester (MDT) bySchlumberger, use straddle packers in a normal operation.

Although the use of straddle packers may significantly improve the flowrate over single or dual-probe assemblies because fluid is beingcollected from the entire isolated area, it also has several importantlimitations that adversely affect its application in certain reservoirconditions. For example, it is generally a practice in the oil and gasindustry to drill boreholes large enough to accommodate different typesof testing, logging, and pumping equipment; therefore, a typical size ofa borehole can be as much as 50 cm in diameter. Since the diameter of atypical formation-testing tool ranges from 10 cm to 15 cm and aninflated packer can increase this range approximately by an additional10 cm, the packers may not provide sufficient isolation of the sampledzone. As a result, sufficient pressure may not be established in thezone of interest to draw fluids from the formation, and drilling mudcirculating in the borehole may also be pumped into the tool.

Furthermore, while straddle packers are effective in many applications,they present operational difficulties that cannot be ignored. Theseinclude a limitation on the number of pressure tests before the straddlepackers deteriorate, temperature limitations, differential pressurelimitations (drawdown versus hydrostatic), and others. Another potentialdrawback of straddle packers includes a limited expansion ratio (i.e.,out-of-round or ovalized holes).

A very important limitation of testing using straddle packers is thatthe testing time is invariably increased due to the need to inflate anddeflate the packers. Other limitations that can be readily recognized bythose of skill in the art include increased pressure stabilization—largewellbore storage factor, difficulty in testing a zone just above or justbelow a washout (i.e., packers would not seal); hole size limitations ofthe type discussed above, and others. Notably, straddle packers are alsosusceptible to gas permeation and/or rubber vulcanizing in the presenceof certain gases.

Accordingly, there is a need to provide a downhole formation testingsystem that combines both the pressure-testing capabilities of dualprobe assemblies and the large exposure volume of straddle packers,without the attending deficiencies associated with the prior art. Tothis end, it is desirable to provide a system suitable for testing,retrieval and sampling from relatively large sections of a formationalong the surface of a wellbore, thereby improving, inter alia,permeability estimates in formations having heterogeneous matrices suchas laminated, vugular and fractured reservoirs. Additionally, it isdesired that the tool be suitable for use in any typical size boreholes,and be deployable quickly for fast measurement cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention are more fully explained in thefollowing detailed description of the preferred embodiments, and areillustrated in the drawings, in which:

FIGS. 1A and 1B show a typical prior art wireline formation tester witha cup-shaped sealing pad providing point contact with the formation;

FIG. 2 is a graphic illustration of a sample of laminated, fractured andvugular formation, frequently encountered in practical applications;

FIG. 3 is an illustration of a prior art tool using inflatable straddlepackers to stabilize the flow rate into the tool;

FIG. 4 shows a schematic diagram of a modular downhole formation-testingtool, which can be used in accordance with a preferred embodiment incombination with the elongated pad design of the present invention;

FIGS. 5A and 5B show a schematic diagram of a dual-probe tester moduleaccording to a preferred embodiment of the present invention (FIG. 5A)and a cross-section of the elongated sealing pad (FIG. 5B) in oneembodiment;

FIGS. 6A-B, 6C-D, 6E-F, 6G-6H, 6I-6J, 6K, and 6L are schematic diagramsof probe modules according to alternative embodiments of the presentinvention;

FIGS. 7A-F are CAD models and schematics of a sealing pad in accordancewith this invention; FIGS. 7G-H show additional detail about how thescreen and gravel pack probe works in a preferred embodiment of thepresent invention;

FIGS. 8A and 8B show a graphical comparison of an Oval Pad design usedin accordance with the present invention with a prior art InflatablePackers flow area;

FIG. 9 illustrates the determination of the maximum pumpout rate in thecomparison tests between the Oval Pad design prior art InflatablePackers design;

FIG. 10 is a pressure contour plot of an Oval Pad in accordance withthis invention, in a ¼ cross section. This finite element simulationshows how the Oval Pad pressures are distributed in the formation at10.2 cc/sec producing a 100 psi pressure drop from formation pressure.The formation has a 1″ lamination located at the center of the pad;

FIG. 11 is a pressure contour plot of a straddle packer using anaxisymmetric finite element simulation; a 100 psi pressure drop betweenthe straddle packers creates a 26.9 cc/sec flow rate; the formation hasa 1″ lamination centered between the straddle packers;

FIG. 12 is a contour plot similar to the one shown in FIG. 10, but a 1mdarcy homogeneous formation is simulated for the Oval Pad. In thiscase, a 100 psi pressure drop causes the Oval Pad to flow at 0.16cc/sec;

FIG. 13 is similar to FIG. 11 but a 1 mdarcy homogeneous formation issimulated for the Inflatable Packers design;

FIGS. 14 and 15 show the pumping performance (flow rate) differencesbetween the Oval Pad and Inflatable Packers technologies. The advantageof using the Oval Pad design in low permeability zones is that acontrollable pumping rate can be maintained where a probe devicerequires a flow rate that is too low to be measured accurately; and

FIG. 16 shows an elongated sealing pad being refracted without extendingbeyond the periphery of the tester.

DETAILED DESCRIPTION OF THE INVENTION The Modular Fluid Testing Tool

The system of present invention is best suited for use with a modulardownhole formation testing tool, which in a preferred embodiment is theReservoir Description Tool (RDT) by Halliburton. As modified inaccordance with the present invention, the tool is made suitable fortesting, retrieval and sampling along sections of the formation by meansof contact with the surface of a borehole. In accordance with apreferred embodiment illustrated in FIG. 4, the formation-testing tool10 comprises several modules (sections) capable of performing variousfunctions. As shown in FIG. 4, tool 10 may include a hydraulic powermodule 20 that converts electrical into hydraulic power; a probe module30 to take samples of the formation fluids; a flow control module 40regulating the flow of various fluids in and out of the tool; a fluidtest module 50 for performing different tests on a fluid sample; amulti-chamber sample collection module 60 that may contain various sizechambers for storage of the collected fluid samples; a telemetry module70 that provides electrical and data communication between the modulesand an uphole control unit (not shown), and possibly other sectionsdesignated in FIG. 4 collectively as 80. The arrangement of the variousmodules may depend on the specific application and is not consideredherein.

More specifically, the power telemetry section 70 conditions power forthe remaining tool sections. Each section preferably has its ownprocess-control system and can function independently. While section 70provides a common intra-tool power bus, the entire tool string(extensions beyond tool 10 not shown) shares a common communication busthat is compatible with other logging tools. This arrangement enablesthe tool in a preferred embodiment to be combined with other loggingsystems, such as a Magnetic Resonance Image Logging (MRIL.dagger.) orHigh-Resolution Array Induction (HRAI.dagger.) logging systems.

Formation-testing tool 10 is conveyed in the borehole by wireline (notshown), which contains conductors for carrying power to the variouscomponents of the tool and conductors or cables (coaxial or fiber opticcables) for providing two-way data communication between tool 10 and anuphole control unit. The control unit preferably comprises a computerand associated memory for storing programs and data. The control unitgenerally controls the operation of tool 10 and processes data receivedfrom it during operations. The control unit may have a variety ofassociated peripherals, such as a recorder for recording data, a displayfor displaying desired information, printers and others. The use of thecontrol unit, display and recorder are known in the art of well loggingand are, thus, not discussed further. In a specific embodiment,telemetry module 70 may provide both electrical and data communicationbetween the modules and the uphole control unit. In particular,telemetry module 70 provides high-speed data bus from the control unitto the modules to download sensor readings and upload controlinstructions initiating or ending various test cycles and adjustingdifferent parameters, such as the rates at which various pumps areoperating.

Flow control module 40 of the tool preferably comprises a double actingpiston pump, which controls the formation fluid flow from the formationinto flow line 15 via probes 32 a and 32 b. The pump operation isgenerally monitored by the uphole control unit. Fluid entering theprobes 32 a and 32 b flows through the flow line 15 and may bedischarged into the wellbore via outlet 44. A fluid control device, suchas a control valve, may be connected to flow line 15 for controlling thefluid flow from the flow line 15 into the borehole. Flow line fluids canbe preferably pumped either up or down with all of the flow line fluiddirected into or though pump 42. Flow control module 40 may furtheraccommodate strain-gauge pressure transducers that measure an inlet andoutlet pump pressures.

The fluid testing section 50 of the tool contains a fluid testingdevice, which analyzes the fluid flowing through flow line 15. For thepurpose of this invention, any suitable device or devices may beutilized to analyze the fluid. For example, Halliburton Memory Recorderquartz gauge carrier can be used. In this quartz gauge the pressureresonator, temperature compensation and reference crystal are packagedas a single unit with each adjacent crystal in direct contact. Theassembly is contained in an oil bath that is hydraulically coupled withthe pressure being measured. The quartz gauge enables measurement ofsuch parameters as the drawdown pressure of fluid being withdrawn andfluid temperature. Moreover, if two fluid testing devices 52 are run intandem, the pressure difference between them can be used to determinefluid viscosity during pumping or density when flow is stopped.

Sample collection module 60 of the tool may contain various sizechambers for storage of the collected fluid sample. Chamber section 60preferably contains at least one collection chamber, preferably having apiston that divides chamber 62 into a top chamber 62 a and a bottomchamber 62 b. A conduit is coupled to bottom chamber 62 b to providefluid communication between bottom chamber 62 b and the outsideenvironment such as the wellbore. A fluid flow control device, such asan electrically controlled valve, can be placed in the conduit toselectively open it to allow fluid communication between the bottomchamber 62 b and the wellbore. Similarly, chamber section 62 may alsocontain a fluid flow control device, such as an electrically operatedcontrol valve, which is selectively opened and closed to direct theformation fluid from the flow line 15 into the upper chamber 62 a.

The Probe Section

Probe module 30, and more particularly the sealing pad, which is thefocus of this invention, comprises electrical and mechanical componentsthat facilitate testing, sampling and retrieval of fluids from theformation. As known in the art, the sealing pad is the part of the toolor instrument in contact with the formation or formation specimen. Inaccordance with this invention a probe is provided with at least oneelongated sealing pad providing sealing contact with a surface of theborehole at a desired location. Through one or more slits, fluid flowchannel or recesses in the sealing pad, fluids from the sealed-off partof the formation surface may be collected within the tester through thefluid path of the probe. As discussed in the next section, therecess(es) in the pad is also elongated, preferably along the axis ofthe elongated pad, and generally is applied along the axis of theborehole. In a preferred embodiment, module 30 is illustrated in FIGS.5A and 5B.

In the illustrated embodiment, one or more setting rams (shown as 31 aand 31 b) are located opposite probes 32 a and 32 b of the tool. Rams 31a and 31 b are laterally movable by actuators placed inside the probemodule 30 to extend away from the tool. Pretest pump 33 preferably isused to perform pretests on small volumes of formation fluid. Probes 32a and 32 b may have high-resolution temperature compensated strain gaugepressure transducers (not shown) that can be isolated with shut-invalves to monitor the probe pressure independently. Pretest piston pump33 also has a high-resolution, strain-gauge pressure transducer that canbe isolated from the intra-tool flow line 15 and probes 32 a and 32 b.Finally, in a preferred embodiment the module may include a resistance,optical or other type of cell (not shown) located near probes 32 a and32 b to monitor fluid properties immediately after entering eitherprobe.

Probe module 30 generally allows retrieval and sampling of formationfluids in sections of a formation along the longitudinal axis of theborehole. As shown in FIG. 5A, module 30 comprises two or more probes(illustrated as 32 a and 32 b) preferably located in a range of 5 cm to100 cm apart. Each probe has a fluid inlet approximately 1 cm to 5 cm indiameter, although other sizes may be used as well in differentapplications. The probes in a preferred embodiment are laterally movableby actuators placed inside module 30 to extend the probes away from thetool.

As shown in FIG. 5A and illustrated in further detail in FIG. 5B,attached to the probes in a preferred embodiment is an elongated sealingpad 34 for sealing off a portion on the side wall of a borehole. Pad 34is removably attached in a preferred embodiment for easy replacement,and is discussed in more detail below. The recess of the sealing padshown in FIG. 5B measures 9.00″ in length and 1.75″ in width.

FIGS. 6A-B, 6C-D and 6E-F are schematic diagrams of probe modulesaccording to alternative embodiments of the present invention. In thefirst alternative design shown in FIG. 6A, a large sealing pad 34 (shownin FIG. 6B) is supported by a single hydraulic piston 32. The secondalternative design (shown in FIG. 6C) shows two elongated (FIG. 6D)sealing pads supported by a set of pistons 32 a and 32 b. A design usingtwo elongated pads on the same tool may have the advantage of providinga greater longitudinal length that could be covered with two pads versusone. It will be apparent that other configurations may be used inalternate embodiments. FIG. 6F illustrates an embodiment in which therecess in the pad is divided into two parts 36 a and 36 a correspondingrespectively to fluid flow into the individual probes, as shown in FIG.6E.

In particular, one such embodiment, which is not illustrated in thefigures, is to use an elongated sealing pad attached to multiplehydraulic rams. The idea is to use the rams not only to deploy the padbut also to create separate flow paths. Carrying this idea a bitfurther, an articulated elongated pad could be supported by severalhydraulic rams, the extension of which can be adjusted to cover agreater length of borehole. A potential benefit of articulating the padis to make it more likely to conform to borehole irregularities, and toprovide improved sealing contact.

Another alternative embodiment is to use pads attached to hydraulic ramsthat are not aligned longitudinally, as shown in FIGS. 5A, 6A, 6C, and6E. In such embodiments, an array of elongated pads with differentangular deployment with respect to the borehole may be used (i.e.,diagonally opposite, or placed at various angles with respect to theprobe). An expected benefit of an array of pads is that more boreholecoverage could be achieved making the device practically equivalent, orin some instances even superior to the straddle packer. In particular,the pads may be arranged in an overlapping spiral fashion around thetool making the coverage continuous.

FIGS. 6G and 6H are schematic diagrams of probe or tester modules 30according to alternative embodiments of the present disclosure. A largesealing pad 34 is curved to follow the radius of the wellbore 13 and maybe curved and extend circumferentially in one axial plane or may becurved and extend circumferentially and axially about an outer surfaceof the probe module 30. The elongated sealing pad 34 is supported by oneor more hydraulic rams 31 that deploy the pad 34 toward the surface 13 aof the wellbore 13 and may create separate flow paths where sealing pad34 includes more than one slit or recess 36 for drawing of formationfluids into the probes 30. In the present embodiment shown in FIGS. 6Gand 6H, the probe module 30 includes three pads 34 a, 34 b, 34 c spacedcircumferentially about the outer surface of the probe module 30, andeach pad is supported by two hydraulic rams 31 a, 31 b; however, inother embodiments, three or more rams 31 may be used. A first end 34′ ofeach pad 34 may or may not circumferentially overlap a second end 34″ ofan adjacent pad 34. The pads 34 a, 34 b, 34 c shown in FIG. 6H do notoverlap; however, pad length, angle of orientation, and positioning onthe probe module 30 may be adjusted in any combination to allow pads 34to overlap circumferentially. In addition, one ram 31 a may be actuatedor extended independently from the other rams 31 b, 31 c such that oneram 31 a is extended a different amount than one or more of the otherrams 31 b, 31 c to make the pad 34 more likely to conform to wellboreirregularities. For example, if one portion of the wellbore wall has alarger diameter than an adjacent portion of the wellbore wall, the pad34 can accommodate the variation in wellbore diameter by extending theram 31 closer to the larger diameter portion further to form a seal.

FIGS. 6I and 6J are schematic diagrams of probe or tester modules 30according to alternative embodiments of the present disclosure. Aplurality of pads 34 is disposed about a probe module 30 and supportedon a series of bands 46 that are interwoven or braided with one anotherto form a banded assembly 41. In FIGS. 6I and 6J, four pads 34 a, 34 b,34 c, 34 d are shown spaced circumferentially about the probe module 30;however, in other embodiments, one or more pads 34 may be used. The pads34 may extend outward away from, be flush with, or be recessed withinthe band assembly 41. In an embodiment, the pads 34 may be flush toslightly extended or bulged outward such that when the band assembly 41is under pressure, the pads 34 become flush with the band assembly 41.

Actuators 51 are disposed at each end of the banded assembly 41 to drivethe bands 46, and thus the pads 34, either outward toward the wellborewall or inward toward the probe module 30. Actuators 51 may be anysuitable device known in the art capable of linear motion, rotationalmotion, or both, including, but not limited to, hydraulic or electricrings. In an embodiment, actuators 51 may be rams that compress the twoends of the band assembly 41 toward each other causing the band assembly41 to extend or bulge outward in the middle toward the wellbore. Inanother embodiment, actuators 51 may use a screw action to twist orrotate one or both of the two ends of the band assembly 41 causing theband assembly 41 to expand or bulge outward in the middle toward thewellbore wall. The bands 46 may further be preferentially twisted orbiased in one direction and then actuated by turning one end of the bandassembly 41 in another direction. In another embodiment, actuators 51may use a combination of compression and torsion to expand the bandassembly 41 toward the wellbore wall or contract the band assembly 41,and thus the pads 34, away from the wellbore wall and toward the probemodule 30. Fluid samples may be taken through a conduit or hose thatmay, but need not, be flexible.

The band assembly 41 may be hydraulically balanced and open to thewellbore, or the band assembly 41 may further include a film or bladder48 disposed on either the outer or inner surface of the band assembly 41to provide an impermeable coating. For example, the bladder 48 may befitted inside the banded assembly 41 such that the bladder 48 isdisposed between the banded assembly 41 and the probe module 30, or thebladder 48 may be fitted over the banded assembly 41 such that thebladder 48 is disposed between the banded assembly 41 and the wellbore.In an embodiment, the bladder 48 is fitted over the band assembly 41,such that removal of fluid from the volume behind the bladder 48 cangenerate the force to unset the tool. In another embodiment, the bandassembly 41 may be hydraulically sealed against the tool 10 by thebladder 48, and fluid may be drawn into the inner portion of the bandassembly 41. When the bladder 48 is disposed on the interior of the bandassembly 41, the fluid and bladder 48 form a bag or seal allowing theband assembly 41 to also be used for communication uphole. The bladder48 may be made from any pliable material known in the art including, butnot limited to, an impermeable elastomer, and Kevlar.

FIG. 6K is a schematic diagram of probe or tester modules 30 accordingto alternative embodiments of the present disclosure. The probe module30 includes a plurality of pads 34, and each pad 34 is disposed on aflexible bow 37. Rams 31 may be disposed at one or both ends of the bows37 to actuate the bows 37. In particular, rams 31 may be placed at bothends of the bow 37, or one end of the bows 37 (either upper or lower endin relation to the surface) may be fixed with rams 31 disposed at theother end (either lower or upper end in relation to the surface). Thefixed end of the bows 37 may further include fluid flow and sensingconnections. The rams 31 may apply force to compress or move the ends ofthe bows 37 closer together thereby extending or moving the pads 34closer to the wellbore wall or the rams may apply force to move the endsof the bows 37 away from one another thereby retracting or moving thepads 34 closer to the probe module 30. Fluid samples may be takenthrough a conduit or hose that may, but need not, be flexible. Forexample, a limited range of rotation fluid joint or fluid swivel may beused to collect fluid.

FIG. 6L is a schematic diagram of probe or tester modules 30 accordingto alternative embodiments of the present disclosure. A plurality ofpads 34 is disposed on an expandable or inflatable sleeve 51. The pads34 may be oriented longitudinally or at an angle between 0 degrees and180 degrees with respect to the longitudinal axis of the probe or tester30. The pads 34 may further be spaced circumferentially about the probemodule 30 such that the center of mass of the pads 34 is centered aboutthe central axis of the tester or probe module 30. Sleeve 51 may behydraulically inflatable to extend the pads 34 toward the wellbore walland hydraulically deflated to retract the pads 34 back toward the probemodule 30.

In the present embodiment, the probe module 30 includes three pads 34 a,34 b, 34 c spaced circumferentially about the outer surface of the probemodule 30; however, in other embodiments, two or four or more pads 34may be used. A first end 34′ of each pad 34 may or may notcircumferentially overlap a second end 34″ of an adjacent pad 34. Thepads 34 a, 34 b, 34 c shown in FIG. 6L do not overlap; however, padlength, angle of orientation, and positioning on the probe module 30 maybe adjusted in any combination to allow pads 34 to overlapcircumferentially. In addition, because sleeve 51 is flexible, when oneend 34′, 34″ of a pad 34 contacts the wellbore wall before the other end34″, 34′ of the pad 34 due to an irregularity in the wellbore wall, thesleeve 51 may be further inflated until the other end 34″, 34′ of thepad 34 is also in contact with the wellbore wall to make the pad 34 morelikely to conform to wellbore irregularities. Thus, an amount ofextension of one elongated sealing pad 34 may be different from anamount of extension of one of the other elongated sealing pads 34. Forexample, if one portion of the wellbore wall has a larger diameter thanan adjacent portion of the wellbore wall, the pad 34 can accommodate thevariation in wellbore diameter by expanding the sleeve 51 further untila seal is formed.

In alternative embodiments, better design flexibility can be providedusing redundancy schemes, in which variable size or property pads,attached to different numbers of extension elements of a probe, andusing combinations of different screens, filtering packs, and others maybe used.

Alternative designs are clearly possible and are believed to be usedinterchangeably with the specific designs illustrated in thisdisclosure.

The Sealing Pad

An important aspect of the present invention is the use of one or moreelongated sealing pads with a slot or recess cut into the face of thepad(s), as shown in a preferred embodiment in FIG. 5A. The slot in thepad is preferably screened and gravel or sand packed, depending onformation properties. In operation, sealing pad 34 is used tohydraulically seal off an elongated portion along a surface of theborehole, typically disposed along the axis of the borehole.

FIG. 5A illustrates the face of an elongated sealing pad in accordancewith one embodiment of this invention. In this embodiment, sealing pad34 is preferably at least twice as long as the distance between probes32 a and 32 b and, in a specific embodiment, may be dimensioned to fit,when not in use, into a recess provided on the body of probe module 30without extending beyond the periphery of the tool. As explained above,sealing pad 34 provides a large exposure area to the formation fortesting and sampling of formation fluids across laminations, fracturesand vugs.

Sealing pad 34 is preferably made of elastomeric material, such asrubber, compatible with the well fluids and the physical and chemicalconditions expected to be encountered in an underground formation.Materials of this type are known in the art and are commonly used instandard cup-shaped seals.

With reference to FIG. 5B, sealing pad 34 has a slit or recess 36 cuttherein to allow for drawing of formation fluids into the probes. Slit36 preferably extends longitudinally the length of sealing pad 34 endinga few centimeters before its edges. The width of slit 36 is preferablygreater than, or equal to, the diameter of the inlets. The depth of slit36 is preferably no greater than the depth of sealing pad 34. In apreferred embodiment, sealing pad 34 further comprises a slotted screen38 covering slit 36 to filter migrating solid particles such as sand anddrilling debris from entering the tool. Screen 38 is preferablyconfigured to filter out particles as small as a few millimeters indiameter. In a preferred embodiment, sealing pad 34 is further gravel orsand packed, depending on formation properties, to ensure sufficientsealing contact with the borehole wall.

FIGS. 7A-F are CAD models and schematics of a sealing pad in accordancewith this invention. FIG. 7A shows a 3D view of the elongated sealingpad. FIG. 7A shows rigid base 43 and elastomeric pad 34. Recess 36fitted with steel aperture 39 is also shown. FIGS. 7B, 7C, and 7E showfront, top, and side views of the structures shown in FIG. 7A. The widthof the structure, as seen in FIG. 7E is 4.50″ and the radius of thecurvature is 4.12. FIGS. 7D and 7F show longitudinal and transversecross-sectional views. In the embodiment shown in FIG. 7D, the length ofrecess 36 surrounded by steel aperture 39 is 9.00″ and the length ofelastomeric pad 34 is 11.45″. In FIG. 7F, the width of recess 36surrounded by aperture 39 is 1.75″. It should be noted that alldimensions in the figures are approximate and may be varied inalternative embodiments.

In a preferred embodiment, the pad is provided with a metal cup-likestructure that is molded to the rubber to facilitate sealing. Othergeometries are possible but the basic principle is to support the rubbersuch that it seals against the borehole but is not allowed to be drawninto the flow area. A series of slots or an array of holes could also beused in alternative embodiments to press against the borehole and allowthe fluid to enter the tool while still maintaining the basic elongatedshape.

FIGS. 7G-H show additional detail about how the screen and gravel packprobe 32 works in a preferred embodiment of the present invention. Asillustrated, in this embodiment the elongated sealing pad 34 is attachedto a hydraulic ram and the probe with a slotted screen at one of theinlet openings. The alignment of sealing pad 34 with respect to probe 32is ensured by sliding tongue 47 into groove 45 (shown in FIG. 7F.)Notice that the fluids are directed through the screen slots into anannular area, which connects to a flow line in the tool. When thehydraulic ram deploys the Oval Pad against the well bore, theelastomeric material of the pad is compressed. The hydraulic systemcontinues to apply an additional force to the probe assembly, causing itto contact the steel opening aperture 39 of the elongated pad.Specifically, extendable probe assembly 59 shown in its retractedposition in FIG. 7H pushes against steel aperture 39, as shown in FIG.7G. Therefore, it will be appreciated that the steel aperture 39 ispressed against the borehole wall with greater force than the rubber.This system of deployment insures that the steel aperture 39 keeps therubber from extruding and creates a more effective seal in a preferredembodiment. When the elongated pad 34 is retracted, the probe screenassembly is retracted and a wiper cylinder pushes mudcake or sand fromthe screen area. In alternative embodiments this screen can be replacedwith a gravel pack type of material to improve the screening of veryfine particles into the tool's flowline.

In another embodiment of the invention, the sealing pad design may bemodified to provide isolation between different probes (such as 32 a and32 b in FIG. 5A), which may be useful in certain test measurements.Thus, in pressure gradient tests, in which formation fluid is drawn intoone probe and changes in pressure are detected at the other probe,isolation between probes is needed to ensure that there is no directfluid flow channel outside the formation between the probe and thepressure sensor; the tested fluid has to flow though the formation.

Accordingly, such isolation between the probes 32 a and 32 b may beaccomplished in accordance with the present invention by dividing slit36 of the sealing pad, preferably in the middle, into two portions 36 aand 36 b. Slits 36 a and 36 b may also be covered with a slottedscreen(s) 38 to filter out fines. As noted in the preceding section,isolation between the probes 32 a and 32 b may also be accomplished byproviding probes 32 a and 32 b with separate elongated sealing pads 34 aand 34 b respectively. As before, each pad has a slit covered by aslotted screen to filter out fines. One skilled in the art shouldunderstand that in either of the above-described aspects of theinvention the probe assembly has a large exposure volume sufficient fortesting and sampling large elongated sections of the formation.

Various modifications of the basic pad design may be used in differentembodiments of the invention without departing from its spirit. Inparticular, in designing a sealing pad, one concern is to make it longenough so as to increase the likelihood that multiple layers in alaminated formation may be covered simultaneously by the fluid channelprovided by the slit in the pad. The width of the pad is likely to bedetermined by the desired angular coverage in a particular boreholesize, by the possibility to retract the pad within the tester module asto reduce its exposure to borehole conditions, and others. In general,in the context of this invention an elongated sealing pad is one thathas a fluid-communication recess that is longer in one dimension(usually along the axis of the borehole).

It should be noted that various embodiments of a sealing pad may beconceived in accordance with the principles of this invention. Inparticular, it is envisioned that a pad may have more than one slit,that slits along the face of the pad may be of different lengths, andprovide different fluid communication channels to the associated probesof the device.

Finally, in one important aspect of the invention it is envisioned thatsealing pads be made replaceable, so that pads that are worn or damagedcan easily be replaced. In alternate embodiments discussed above,redundancy may be achieved by means of more than one sealing padproviding fluid communication with the inlets of the tester.

Operation of the Tool

With reference to the above discussion, formation-testing tool 10 ofthis invention may be operated in the following manner: in a wirelineapplication, tool 10 is conveyed into the borehole by means of wireline15 to a desired location (“depth”). The hydraulic system of the tool isdeployed to extend rams 31 a and 31 b and sealing pad(s) includingprobes 32 a and 32 b, thereby creating a hydraulic seal between sealingpad 34 and the wellbore wall at the zone of interest. Once the sealingpad(s) and probes are set, a pretest is generally performed. To performthis pretest, a pretest pump may be used to draw a small sample of theformation fluid from the region sealed off by sealing pad 34 into flowline 15 of tool 10, while the fluid flow is monitored using pressuregauge 35 a or 35 b. As the fluid sample is drawn into the flow line 50,the pressure decreases due to the resistance of the formation to fluidflow. When the pretest stops, the pressure in the flow line 15 increasesuntil it equalizes with the pressure in the formation. This is due tothe formation gradually releasing the fluids into the probes 32 a and 32b.

Formation's permeability and isotropy can be determined, for example, asdescribed in U.S. Pat. No. 5,672,819, the content of which isincorporated herein by reference. For a successful performance of thesetests isolation between two probes is preferred, therefore,configuration of probe module 30 shown in FIG. 6b or with a divided slitis desired. The tests may be performed in the following manner: Probes32 a and 32 b are extended to form a hydraulically sealed contactbetween sealing pads 34 a and 34 b. Then, probe 32 b, for example, isisolated from flow line 15 by a control valve. Piston pump 42, then,begins pumping formation fluid through probe 32 a. Since piston pump 42moves up and down, it generates a sinusoidal pressure wave in thecontact zone between sealing pad 34 a and the formation. Probe 32 b,located a short distance from probe 32 a, senses properties of the waveto produce a time domain pressure plot which is used to calculate theamplitude or phase of the wave. The tool then compares properties of thesensed wave with properties of the propagated wave to obtain values thatcan be used in the calculation of formation properties. For example,phase shift between the propagated and sensed wave or amplitude decaycan be determined. These measurements can be related back to formationpermeability and isotropy via known mathematical models.

It should be understood by one skilled in the art that probe module 30enables improved permeability and isotropy estimation of reservoirshaving heterogeneous matrices. Due to the large area of sealing pad 34,a correspondingly large area of the underground formation can be testedsimultaneously, thereby providing an improved estimate of formationproperties. For example, in laminated or turbidite reservoirs, in whicha significant volume of oil or a highly permeable stratum is oftentrapped between two adjacent formation layers having very lowpermeabilities, elongated sealing pad 34 will likely cover several suchlayers. The pressure created by the pump, instead of concentrating at asingle point in the vicinity of the fluid inlets, is distributed alongslit 36, thereby enabling formation fluid testing and sampling in alarge area of the formation hydraulically sealed by elongated sealingpad 34. Thus, even if there is a thin permeable stratum trapped betweenseveral low-permeability layers, such stratum will be detected and itsfluids will be sampled. Similarly, in naturally fractured and vugularformations, formation fluid testing and sampling can be successfullyaccomplished over matrix heterogeneities. Such improved estimates offormation properties will result in more accurate prediction ofhydrocarbon reservoir's producibility.

To collect the fluid samples in the condition in which such fluid ispresent in the formation, the area near sealing pad 34 is flushed orpumped. The pumping rate of the double acting piston pump 42 may beregulated such that the pressure in flow line 15 near sealing pad 34 ismaintained above a particular pressure of the fluid sample. Thus, whilepiston pump 42 is running, the fluid-testing device 52 can measure fluidproperties. Device 52 preferably provides information about the contentsof the fluid and the presence of any gas bubbles in the fluid to thesurface control unit 80. By monitoring the gas bubbles in the fluid, theflow in the flow line 15 can be constantly adjusted so as to maintain asingle-phase fluid in the flow line 15. These fluid properties and otherparameters, such as the pressure and temperature, can be used to monitorthe fluid flow while the formation fluid is being pumped for samplecollection. When it is determined that the formation fluid flowingthrough the flow line 15 is representative of the in situ conditions,the fluid is then collected in the fluid chamber 62.

When tool 10 is conveyed into the borehole, the borehole fluid entersthe lower section of fluid chamber 62 b. This causes piston 64 to moveinward, filling bottom chamber 62 b with the borehole fluid. This isbecause the hydrostatic pressure in the conduit connecting bottomchamber 62 b and a borehole is greater than the pressure in the flowline 15. Alternatively, the conduit can be closed and by an electricallycontrolled valve and bottom chamber 62 b can be allowed to be filledwith the borehole fluid after tool 10 has been positioned in theborehole. To collect the formation fluid in chamber 62, the valveconnecting bottom chamber 62 a and flow line 15 is opened and pistonpump 42 is operated to pump the formation fluid into flow line 15through the inlets in slit 36 of sealing pad 34. As piston pump 42continues to operate, the flow line pressure continues to rise. When theflow line pressure exceeds the hydrostatic pressure (pressure in bottomchamber 62 b), the formation fluid starts to fill in top chamber 62 a.When the upper chamber 62 a has been filled to a desired level, thevalves connecting the chamber with both flow line 15 and the boreholeare closed, which ensures that the pressure in chamber 62 remains at thepressure at which the fluid was collected therein.

The above-disclosed system for the estimation of relative permeabilityhas significant advantages over known permeability estimationtechniques. In particular, borehole formation-testing tool 10 combinesboth the pressure-testing capabilities of the known probe-type tooldesigns and large exposure volume of straddle packers. First, tool 10 iscapable of testing, retrieval and sampling of large sections of aformation along the axis of the borehole, thereby improving, inter alia,permeability estimates in formations having heterogeneous matrices suchas laminated, vugular and fractured reservoirs.

Second, due to the tool's ability to test large sections of theformation at a time, the testing cycle time is much more efficient thanthe prior art tools. Third, it is capable of formation testing in anytypical size borehole.

In an important aspect of the invention, the use of the elongatedsealing pad of this invention for probing laminated or fracturereservoir conditions may be optimized by first identifying theprospective laminated zones with conventional, high-resolution wirelinelogs. In a preferred embodiment, the identification of such zones may bemade using imaging tools, such as electric (EMI) or sonic (CAST-V)devices, conventional dipmeter tools, microlog tools, ormicro-spherically focused logs (MSFL). As an alternative, prospectivelayered zones can be identified using high-resolution resistivity logs(HRI or HRAI), or nuclear logs with high resolution (EVR). Other toolsor methods for identifying thin-bed laminated structures will beapparent to those of skill in the art and are not discussed in furtherdetail.

In a first embodiment, the identification of the laminate structure bestsuitable for testing, using the device and methods of this invention, isdone by running the identifying logging tool first and then rapidlypositioning the probes of the fluid tester in a sealing engagement witha surface of the borehole located by the logging tool. In thealternative, the fluid tester may be used in the same run as the loggingdevice, to use the rapid-deployment ability of the Oval Pad design ofthe invention.

Advantages of the Proposed Approach

Some of the primary advantages to the novel design approach usingelongated pads are as follows:

1. enables placement of an isolated flow path across an extendedformation face along the borehole trajectory;2. provides the ability to expose a larger portion of the formation faceto pressure measurements and sample extraction;3. potential benefits in laminated sequences of sand/silt/shale, wherepoint-source probe measurements may not connect with permeable reservoirporosity;4. potential benefit in formations subject to localized inconsistenciessuch as intergranular cementation (natural or induced), vugular porosity(carbonates and volcanics) and sectors encountering lost circulationmaterials;5. ability to employ variable screen sizes and resin/gravel selectivity;6. stacked for multiple redundancy or variable configuration of multipleprobe section deployments, including standard and gravel pack probes;7. reduced risk of sticking as may be encountered with packer type pumptester devices;8. faster cleanup and sample pumpout times under larger differentialpressures;9. easily adapted to existing wireline, LWD or DST technologies;10. quicker setting, testing and retracting times over straddle packers;11. ability to take multiple pressure tests and samples in a singletrip.

Persons skilled in the art will recognize other potential advantages,including better seating and isolation of the pad versus straddlepackers, ability to perform conventional probe type testing procedures,and others.

Applications and Comparison Examples

As noted above, the tester devices and methods in accordance with thepresent invention are suitable for use in a wide range of practicalapplications. It will be noted, however, that the advantages of thenovel design are most likely to be apparent in the context ofunconventional reservoirs, with a particular interest in laminatedreservoirs. Thus, reservoir types, the exploration of which is likely tobenefit from the use of the systems and methods of this invention,include, without limitation, turbidites and deepwater sands, vugularformations, and naturally fractured reservoirs, in which the approachused in this invention will allow for sampling (pressure and fluid) of alarger section of the formation along the axis of the tool and borehole.

Importantly, in accordance with a preferred embodiment of the invention,MWD testing would benefit from the use of the device in accordance withthis invention, for both pressure testing (i.e., formation pressure andmobility) as well as sampling. It is known that a probe device must flowat less than 0.1 cc/sec, which means the pump is close to 4000 psipressure differential. It is difficult to devise a flow control systemto control a rate below 0.1 cc/sec, and even if this were possible therewould still be a considerable error in the mobility measurement.

The table below summarizes finite element simulations of a test designusing the novel elongated pad (“Oval Pad”) approach of this inventionused with the Reservoir Description Tool (“RDT”) by Halliburton, ascompared with a simulation of a prior art tool using inflatable straddlepackers (the “Inflatable Packers” design). The prior art simulationsillustrated here are for the Modular Formation Dynamics Tester (“MDT”)by Schlumberger.

The two tester configurations are compared in FIGS. 8A and 8B, where theOval Pad of this invention (RTD Straddle Pad) is represented in FIG. 8Aas a slot area 1.75″ wide and 9.0″ long, while the Inflatable Packersflow area of the prior art (MDT Inflatable Straddle Packers) is modeledas a cylinder 8.5″ in diameter and 39″ long as shown in FIG. 8B. The 9″oval pad was selected for comparison against the 39″ straddle packer as9″ is a preferred dimension in a specific embodiment, and the 39″straddle packer represents typical prior technology.

It will be noticed that while the prior art Inflatable Packers designhas a full 360.degree. (26.7″) coverage, the Oval Pad design, inaccordance with this invention, has an equivalent of only 26.7.degree.(1.75″) coverage angle. Two flow rates are predicted for eachconfiguration, as illustrated in FIG. 9. The first flow rate isdetermined at a fixed 100 psi pressure pumping differential. The secondflow rate is the maximum flow rate for each system, which considers therespective pump curves and a 1000 psi hydrostatic overbalance. Asillustrated in the figure, the formation pumpout rate varies linearlyand the maximum flow rate is determined by calculating the intersectionof the formation rate curve with the pump curve, which is also nearlylinear.

The first set of simulations consider a low permeability zone (1 mDarcy)with a single 1″ wide high-permeability lamination (1 Darcy)intersecting the vertical spacing. The same formation model is exposedto the Oval Pad design of this invention and the prior art InflatablePackers flow area. As illustrated in FIGS. 10 and 11, the Oval Padproduces at 10.2 cc/sec and the Inflatable Packers design produces 26.9cc/sec with a 100 psi pressure differential.

The maximum pumping rate of 38.8 cc/sec is determined for the Oval Paddesign of this invention, assuming a conservative pump curve for theflow control pump-out section (FPS) of the tool and an overbalance of1000 psi. The maximum pumping rate for the prior art straddle packerdesign is estimated at 29.1 cc/sec, which estimate is determined using ahigh-end pump curve estimate for the MDT tool. It is notable thatdespite the increased vertical spacing and exposed area of the straddlepacker's design, its maximum flow rate is lower for the laminated zonecase. This result is likely due to the MDT reduced pumping ratecapabilities as compared to the pump-out module of the RDT tool.

TABLE-US-00001 Radial Flow Rate Maximum Rate Vertical Packer EquivalentLamination (cc/sec) (cc/sec) Spacing Equivalent Width 1 Darcy @ 100 psi@ 1000 psi Simulation (inches) Angle (inches) 1″ Thick differentialoverbalance RDT Oval Pad 9.00 23.6.degree. 1.75 Yes 10.2 38.8*MDTInflatable 39.00 360.0.degree. 26.7 Yes 26.9 29.1.sup..dagger. PackersRDT Oval Pad 9.00 23.6.degree. 1.75 No 0.16 3.8*MDT Inflatable 39.00360.0.degree. 26.7 No 2.1 19.5.sup..dagger. Packers*RDT Pumpout Rateusing 3600 psi @ 0 cc/sec and 0 psi @ 63 cc/sec pump curve (see FIG.2).sup..dagger. MDT Pumpout Rate using 3600 psi @ 0 cc/sec and 0 psi @42 cc/sec pump curve (see FIG. 2)

FIG. 10 is a pressure contour plot of Oval Pad ¼ cross section. Thisfinite element simulation shows how the Oval Pad pressures aredistributed in the formation at 10.2 cc/sec producing a 100 psi pressuredrop from formation pressure. The formation has a 1″ lamination locatedat the center of the pad.

FIG. 11 is a pressure contour plot of a straddle packer using anaxisymmetric finite element simulation. A 100 psi pressure drop betweenthe straddle packers creates a 26.9 cc/sec flow rate. The formation hasa 1″ lamination centered between the straddle packers.

The other case illustrated for comparison is a testing of lowpermeability zones. In particular, the simulations were performed with ahomogeneous 1 mDarcy zone. In this case, as illustrated in FIG. 12, a100 psi pressure drop causes the Oval Pad to flow at 0.16 cc/sec. Thesame pressure drop with Inflatable Packers produces 2.1 cc/sec, asillustrated in FIG. 13. While the difference appears relatively large,it should be considered in the context of the total system pumpingcapabilities. Thus, because of the RDT increased pumping capacity, amaximum pumping of 3.8 cc/sec is determined for the RDT versus 19.5cc/sec for the MDT, reducing any advantage straddle packers may have inlow permeability zones.

Notably, the increased rate for the Inflatable Packers design is lessimportant if one is to consider the time to inflate the packers and voidmost of the contaminating fluid between them. Additionally, it isimportant to consider that the Oval Pad design of this invention shouldmore easily support higher pressure differentials than with theInflatable Packers, as is the case with probes.

The plots in FIGS. 14 and 15 show how the pumping rate and pumping timecompare over a wide range of mobilities, if the pumping system stays thesame. It will be seen that the Inflatable Packer's design generallyenables sampling to occur at a faster rate than the Oval Pad or probedevices. FIG. 15 is an estimate of the pumping time required, assumingthe total volume pumped in order to obtain a clean sample is the samefor each system (i.e., 20 liters). If only the sampling time isconsidered after the Inflatable Packers are deployed it would appearthat using straddle packers allows faster sampling. However, if theinflation and volume trapped between the packers is considered, asexpected, the Oval Pad would obtain a clean sample faster than theInflatable Packers over a large range of mobilities. It is notable thatthe Inflatable Packers design is advantageous only in very low permeablezones. However, it can be demonstrated that if the Oval Pad design isused in a zone that has natural fractures or laminations it would stillsample considerably faster than the prior art Inflatable Packers design.

Yet another important consideration in comparing the Oval Pad to theInflatable Packers designs in practical applications is pressurestabilization. Because of the large volume of fluid filling theinflatable packers and the space between the packers, the storage volumeis many orders of magnitude larger compared with the Oval Pad design ofthis invention. This consideration is an important benefit of the use ofthe design of this invention in transient pressure analysis or simplyfor purposes of obtaining a stable pressure reading.

In reviewing the preceding simulations it is important to note that theyonly illustrate the case of using a single elongated pad. It will beapparent that the use of additional sealing pads will significantlyenhance the comparative advantages of fluid tester designs using theprinciples of this invention.

The foregoing description of the preferred embodiments of the presentinvention has been presented for purposes of illustration andexplanation. It is not intended to be exhaustive nor to limit theinvention to the specifically disclosed embodiments. The embodimentsherein were chosen and described in order to explain the principles ofthe invention and its practical applications, thereby enabling othersskilled in the art to understand and practice the invention. But manymodifications and variations will be apparent to those skilled in theart, and are intended to fall within the scope of the invention, definedby the accompanying claims.

What is claimed is:
 1. A formation tester for testing or samplingformation fluids in a wellbore, the tester comprising: a plurality ofelongated, curved sealing pads having at least one inlet establishingfluid communication between the formation and the interior of thetester, each sealing pad of the plurality of elongated sealing padshaving an outer surface to seal a region along a surface of the wellboreand having at least one elongated recess to establish fluid flow fromthe formation to the at least one opening; and a plurality of rams toextend the elongated sealing pad away from the tester toward theformation and to retract the elongated sealing pad from the formationtoward the tester.
 2. The tester of claim 1, wherein each elongated,curved sealing pad is supported by two or more rams, where extension ofeach ram is independent of extension of the other rams.
 3. The tester ofclaim 2, wherein each elongated, curved sealing pad is curved andextends in one axial plane to follow the radius of the wellbore.
 4. Thetester of claim 2, wherein each elongated, curved sealing pad is curvedand extends circumferentially and axially about an outer surface of thetester.
 5. The tester of claim 2, wherein a first end of one elongated,curved sealing pad circumferentially overlaps a second end of anadjacent elongated, curved sealing pad.
 6. A formation tester fortesting or sampling formation fluids in a wellbore, the testercomprising: a plurality of elongated sealing pads having at least oneinlet establishing fluid communication between the formation and theinterior of the tester, each sealing pad of the plurality of elongatedsealing pads having an outer surface to seal a region along a surface ofthe wellbore and having at least one elongated recess to establish fluidflow from the formation to the at least one opening; an actuator havingat least one flexible member, the plurality of elongated sealing padscoupled to the actuator; and at least one ram coupled to the actuator.7. The tester of claim 6, wherein the actuator comprises a first endopposite a second end, and a plurality of interwoven bands, wherein afirst ram is coupled to the first end and a second ram is coupled to thesecond end to move the first and second ends closer together to extendthe elongated sealing pad away from the tester toward the formation andto move the first and second ends farther apart to retract the elongatedsealing pad away from the formation.
 8. The tester of claim 7, whereinthe actuator is axially movable by the first and second rams.
 9. Thetester of claim 7, wherein the actuator is rotatable by the first andsecond rams.
 10. The tester of claim 7, wherein the plurality ofelongated sealing pads is circumferentially distributed about theactuator.
 11. The tester of claim 7, further comprising an impermeablesealed bladder fitted over the plurality of interwoven bands.
 12. Thetester of claim 6, wherein the actuator comprises a first end opposite asecond end, and a plurality of bows, wherein the at least one ram iscoupled to one of the first and second ends to move the first and secondends closer together to extend the elongated sealing pad away from thetester toward the formation and to move the first and second endsfarther apart to retract the elongated sealing pad away from theformation.
 13. The tester of claim 12, wherein the one of the first andsecond ends of the actuator is axially movable by the at least one ramwhile the other of the first and second ends is fixed.
 14. The tester ofclaim 12, wherein the actuator further comprises a second ram coupled tothe other of the first and second ends.
 15. The tester of claim 12,wherein the plurality of elongated sealing pads are circumferentiallydistributed about the actuator.
 16. A formation tester for testing orsampling formation fluids in a wellbore, the tester comprising: aplurality of elongated sealing pads having at least one inletestablishing fluid communication between the formation and the interiorof the tester, each sealing pad of the plurality of elongated sealingpads having an outer surface to seal a region along a surface of thewellbore and having at least one elongated recess to establish fluidflow from the formation to the at least one opening; and an expandablesleeve, the plurality of elongated sealing pads coupled to theexpandable sleeve at an angle of between 0° and 180° with respect to alongitudinal axis of the tester.
 17. The tester of claim 16, wherein theexpandable sleeve is hydraulically sealed and expanded.
 18. The testerof claim 17, wherein the plurality of elongated sealing pads iscircumferentially distributed about the tester.
 19. The tester of claim18, wherein the center of mass of the plurality of elongated sealingpads is centered about a central axis of the tester.
 20. The tester ofclaim 17, wherein an amount of extension of one elongated sealing pad isdifferent from an amount of extension of at least one other elongatedsealing pad.